Hot-dip galvanized steel sheets and galvannealed steel sheets that have good appearance and adhesion to coating and methods for producing the same (as amended)

ABSTRACT

A hot-dip galvanized steel sheet having a good appearance and good adhesion to a coating, the hot-dip galvanized steel sheet having a composition containing, on a mass basis: C: 0.08% or more and less than 0.20%, Si: 0.1% to 3.0%, Mn: 0.5% to 3.0%, P: 0.001% to 0.10%, Al: 0.01% to 3.00%, and S: 0.200% or less, a remainder being Fe and incidental impurities, wherein the hot-dip galvanized steel sheet includes an internal oxidation layer and a decarburized layer, the internal oxidation layer having a thickness of 5 μm or less, the decarburized layer having a thickness of 20 μm or less, and 50% or more by area of the internal oxidation layer is composed of a Si oxide containing Fe and/or Mn represented by Fe 2X Mn 2-2X SiO Y , wherein X ranges from 0 to 1, and Y is 3 or 4.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT InternationalApplication No. PCT/JP2014/004700, filed Sep. 11, 2014, and claimspriority to Japanese Patent Application No. 2013-188920, filed Sep. 12,2013, the disclosures of each of these applications being incorporatedherein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to hot-dip galvanized steel sheets andgalvannealed steel sheets that are based on Si-containing steel sheetsand have a good appearance and good adhesion to the coating and methodsfor producing the hot-dip galvanized steel sheets and galvannealed steelsheets.

BACKGROUND OF THE INVENTION

In recent years, surface-treated steel sheets produced by rustproofingsteel sheet materials, particularly rustproof hot-dip galvanized steelsheets and galvannealed steel sheets, have been used in the fields ofautomobiles, household electrical appliances, and constructionmaterials.

In general, hot-dip galvanized steel sheets are produced by thefollowing method. First, a slab is subjected to hot rolling, coldrolling, or heat treatment to form a thin steel sheet. The surface ofthe steel sheet is degreased and/or pickled in a pretreatment step.Alternatively, without the pretreatment step, oils on the surface of thesteel sheet are burned in a preheating furnace. The steel sheet is thenheated in a nonoxidizing or reducing atmosphere for recrystallizationannealing. The steel sheet is then cooled in a nonoxidizing or reducingatmosphere to a temperature suitable for coating and is immersed in ahot-dip galvanizing bath without exposed to the air. The hot-dipgalvanizing bath contains a minute amount of Al (approximately 0.1% to0.2% by mass). Thus, the steel sheet is coated and becomes a hot-dipgalvanized steel sheet. Galvannealed steel sheets are produced byheat-treating hot-dip galvanized steel sheets in an alloying furnace.

In recent years, in the automotive field, steel sheet materials have hadhigher performance and reduced weight. Strength reduction resulting fromweight reduction of steel sheet materials is compensated for by theaddition of solid-solution strengthening elements, such as Si and Mn. Inparticular, Si can advantageously reinforce steel without decreasingductility. Thus, Si-containing steel sheets are promising high-strengthsteel sheets. However, the following problems occur in the production ofhot-dip galvanized steel sheets and galvannealed steel sheets based onhigh-strength steel sheets containing large amounts of Si.

As described above, hot-dip galvanized steel sheets are annealed in areducing atmosphere before coating. However, because of its highaffinity for oxygen, Si in steel is selectively oxidized even in areducing atmosphere and forms oxides on the surface of steel sheets.These oxides decrease the wettability of the surface of the steel sheetsand form uncoated areas in a coating operation. Even when uncoated areasare not formed, these oxides decrease the adhesiveness of the coating.

Furthermore, these oxides significantly decrease the alloying speed inan alloying process after hot-dip galvanizing. This greatly decreasesthe production of galvannealed steel sheets. Alloying treatment at hightemperatures for the purpose of high productivity may lower powderingresistance. Thus, it is difficult to achieve efficient production andhigh powdering resistance at the same time. Alloying treatment at hightemperatures makes the retained austenite phase unstable and reduces theadvantage of the addition of Si. Thus, it is very difficult to producehigh-strength hot-dip galvanized steel sheets that have good mechanicalcharacteristics and coating quality at the same time.

Several techniques are disclosed in order to address these problems.Patent Literature 1 discloses a technique for improving the wettabilityof a steel sheet to molten zinc by forming iron oxide on the surface ofthe steel sheet in an oxidizing atmosphere and then forming a reducediron layer on the surface of the steel sheet by reduction annealing.Patent Literature 2 discloses a technique for ensuring high coatingquality by controlling the atmosphere, such as the oxygen concentration,in a preheating operation. Patent Literature 3 discloses a technique ofproducing a hot-dip galvanized steel sheet that has no uncoated area andhas good appearance by dividing the heating zone into three zones A to Cand appropriately controlling the temperature and oxygen concentrationof each of the heating zones to reduce the occurrence of indentationflaws.

CITATION LIST Patent Literature

[PTL 1] Japanese Unexamined Patent Application Publication No. 4-202630

[PTL 2] Japanese Unexamined Patent Application Publication No. 6-306561

[PTL 3] Japanese Unexamined Patent Application Publication No.2007-291498

SUMMARY OF THE INVENTION

In methods of hot-dip galvanizing high-Si-content steel usingoxidation-reduction techniques as described in Patent Literature 1 andPatent Literature 2, although the formation of uncoated areas issuppressed, there is a problem of indentation flaws, which are defectscharacteristic of the oxidation-reduction techniques. A method forcontrolling the temperature and oxygen concentration of A to C heatingzones as described in Patent Literature 3 can be used to produce hot-dipgalvanized steel sheets free of surface defects, such as uncoated areasand indentation flaws. However, the oxide content of steel sheets varieswith production conditions (production planning), and it is difficult toconsistently produce steel sheets. More specifically, even if theheating zone temperatures are maintained constant, under certainproduction conditions (production planning), steel sheets may have aninsufficient oxide content and have an uncoated area.

In view of such situations, it is an object of aspects of the presentinvention to provide hot-dip galvanized steel sheets and galvannealedsteel sheets that have no surface defects and have a good appearance andgood adhesion to the coating and methods for producing the hot-dipgalvanized steel sheets and galvannealed steel sheets. The hot-dipgalvanized steel sheets and galvannealed steel sheets are produced fromhigh-Si-content steel sheets.

When a steel sheet is heated with a direct heating burner in a directheating type furnace, it is known that the amount of oxide formed on thesurface of the steel sheet depends on the furnace temperature or themixing ratio of a combustible gas and a combustion-supporting gas. It isalso known that external oxides are formed on the surface of ferrite (asteel sheet is hereinafter also referred to as ferrite) in a heatingprocess. It is also known that internal oxides are formed inside theinterface between ferrite and external oxides. The external oxidesinclude Fe_(x)Mn_(1-x)O, Fe_(3X)Mn_(3-3X)O₄, and Fe_(2X)M_(2-2X)O₃,wherein X ranges from 0 to 1. The internal oxides include SiO₂ and Sioxides containing Fe and/or Mn represented by Fe_(2X)Mn_(2-2X)SiO_(Y),wherein X ranges from 0 to 1, and Y is 3 or 4. Examples of the Si oxidescontaining Fe and/or Mn represented by Fe_(2X)Mn_(2-2X)SiO_(Y) includeFe₂SiO₄, FeMnSiO₄, Mn₂SiO₄, FeSiO₃, and MnSiO₃. In order to distinguishfrom SiO₂, Si oxides containing Fe and/or Mn represented byFe_(2X)Mn_(2-2X)SiO_(Y) are hereinafter also referred to simply as Sioxides containing Fe and/or Mn.

External oxides thus formed are reduced later in an annealing operationand form a reduced Fe layer on the surface of steel sheets, therebyeffectively improving wettability to Zn coating and suppressing theformation of uncoated areas. Internal oxides, such as SiO₂ and Si oxidescontaining Fe and/or Mn, are effective in decreasing Si activity insteel, suppressing the enrichment of Si on the surface in an annealingoperation, and suppressing the formation of uncoated areas.

The present inventors studied factors that have an influence on theoxidation behavior of high-Si-content steel sheets in addition to thefurnace temperature and gas mixing ratio. When a steel sheet containscarbon, and the combustion atmosphere has low carbon potential,decarbonization occurs simultaneously with oxidation in the steel sheetand lowers oxygen potential in the steel sheet. Consequently, a reactionfrom SiO₂ to Si oxides containing Fe and/or Mn represented byFe_(2X)Mn_(2-2X)SiO_(Y) is promoted inside the interface between ferriteand external oxides.

Because ion diffusion in SiO₂ is slower than ion diffusion in Si oxidescontaining Fe and/or Mn, SiO₂ suppresses outward diffusion of Fe ionsand Mn ions. This suppresses the formation of external oxides, such asFe_(x)Mn_(1-x)O, Fe_(3X)Mn_(3-3X)O₄, and Fe_(2X)Mn_(2-2X)O₃, formed byreactions between Fe and Mn and oxygen. Thus, it is difficult to formexternal oxides required to suppress the formation of uncoated areas byshort-time heating, such as heating with a direct heating burner.Furthermore, internal oxides, such as SiO₂, formed by oxidation areexposed on a portion of the surface of annealed steel sheets not coveredwith a reduced Fe layer. SiO₂ on the surface acts as a starting point ofan uncoated area that repels molten zinc and significantly affects theappearance of the galvanized surface.

In accordance with aspects of the present invention, a reaction fromSiO₂ to Si oxides containing Fe and/or Mn is promoted as the oxygenpotential decreases. Because Si oxides containing Fe and/or Mn havehigher wettability to molten zinc than SiO₂, Si oxides containing Feand/or Mn even exposed on the surface rarely act as starting points ofuncoated areas.

The present inventors studied the heating conditions for stableformation of Si oxides containing Fe and/or Mn in a heat-treatmentprocess including heating in a direct heating type furnace and annealingin a reducing atmosphere. It was found that Si oxides containing 50% ormore by area of Fe and/or Mn can be consistently formed on the ferriteside from the interface between ferrite and a galvanized layer on asteel sheet having a carbon concentration of 0.08% or more and less than0.20% by mass by heating the steel sheet to a final surface temperaturein the range of 600° C. to 800° C. in an atmosphere containing acombustible gas and a combustion-supporting gas, the concentration of COand hydrocarbon gas in the combustible gas being 60% or less by volume,the concentration of O₂ in the combustion-supporting gas ranging from20% to 50% by volume, and then heating the steel sheet at a soakingtemperature in the range of 630° C. to 850° C. in an atmosphere having ahydrogen concentration in the range of 3% to 25% by volume and a watervapor concentration of 0.070% or less by volume and containing N₂ andincidental impurities as a remainder. It was also found that steelsheets produced under these conditions include an internal oxidationlayer having a thickness of 5 μm or less and a decarburized layer havinga thickness of 20 μm or less. The term “internal oxidation layer”, asused herein, refers to a region containing internal oxides in ferrite,more specifically, a region in which at least twice the average ofoxygen peaks observed at a depth in the range of 50 to 60 μm isdetected. The term “decarburized layer”, as used herein, refers to acarbon deficient layer on the ferrite side from the interface betweenferrite and a galvanized layer, more specifically, a region in which thecarbon concentration is less than half the carbon concentration in thebase material.

Aspects of the present invention are based on these findings and aresummarized as follows:

[1] A hot-dip galvanized steel sheet having a good appearance and goodadhesion to a coating, the hot-dip galvanized steel sheet having acomposition containing, on a mass basis: C: 0.08% or more and less than0.20%, Si: 0.1% to 3.0%, Mn: 0.5% to 3.0%, P: 0.001% to 0.10%, Al: 0.01%to 3.00%, and S: 0.200% or less, a remainder being Fe and incidentalimpurities, wherein the hot-dip galvanized steel sheet includes aninternal oxidation layer and a decarburized layer, the internaloxidation layer having a thickness of 5 μm or less on a ferrite sidefrom an interface between ferrite and a galvanized layer, thedecarburized layer having a thickness of 20 μm or less on the ferriteside from the interface between the ferrite and the galvanized layer,and 50% or more by area of the internal oxidation layer is composed of aSi oxide containing Fe and/or Mn represented by Fe_(2X)Mn_(2-2X)SiO_(Y),wherein X ranges from 0 to 1, and Y is 3 or 4.

[2] The hot-dip galvanized steel sheet having a good appearance and goodadhesion to a coating according to [1], further containing Mo: 0.01% to1.00% and/or Cr: 0.01% to 1.00% on a mass basis.

[3] The hot-dip galvanized steel sheet having a good appearance and goodadhesion to a coating according to [1] or [2], further containing atleast one of Nb: 0.005% to 0.20%, Ti: 0.005% to 0.20%, Cu: 0.01% to0.50%, Ni: 0.01% to 1.00%, and B: 0.0005% to 0.010% on a mass basis.

[4] A galvannealed steel sheet having a good appearance and goodadhesion to a coating according to any one of [1] to [3], wherein thegalvanized layer is a galvannealed layer.

[5] A method for producing a hot-dip galvanized steel sheet having agood appearance and good adhesion to a coating, including in sequencethe steps of hot-rolling steel having the composition according to anyone of [1] to [3], cold-rolling the resulting hot-rolled steel sheet;heating the steel sheet to a final surface temperature in the range of600° C. to 800° C. by burning a combustible gas and acombustion-supporting gas with a direct heating burner in a directheating type furnace, the combustible gas containing CO, a hydrocarbongas, and a remainder, the CO and hydrocarbon gas constituting 60% orless by volume in total, the remainder being H₂, N₂, and incidentalimpurities, the combustion-supporting gas containing O₂ and a remainder,the O₂ constituting 20% to 50% by volume, the remainder being N₂ andincidental impurities; heating the steel sheet at a soaking temperaturein the range of 630° C. to 850° C. in an atmosphere having a hydrogenconcentration in the range of 3% to 25% by volume and a water vaporconcentration of 0.070% or less by volume and containing N₂ andincidental impurities as a remainder; and hot-dip galvanizing the steelsheet.

[6] A method for producing a galvannealed steel sheet having a goodappearance and good adhesion to a coating according to [5], furtherincluding alloying the zinc coating after the hot-dip galvanizing.

In accordance with aspects of the present invention, hot-dip galvanizedsteel sheets having no surface defects, such as uncoated areas, andhaving a good appearance and good adhesion to the coating can beconsistently produced from a high-Si-content steel sheet. Although it isgenerally believed that Si is difficult to hot-dip galvanizing, aspectsof the present invention are effective for steel sheets containing 0.1%or more Si or steel sheets based on high-Si-content steel sheets and isuseful as a method for significantly improving the yield in theproduction of high-Si-content hot-dip galvanized steel sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary concentration profile of carbon and oxygen inferrite in a hot-dip galvanized steel sheet according to aspects of thepresent invention.

FIG. 2 summarizes the results listed in Table 2 in Examples; FIG. 2(a)illustrates the results with respect to surface appearance, and FIG.2(b) illustrates the results with respect to the adhesiveness of thecoating.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Exemplary embodiments of the present invention will be further describedbelow.

The composition of steel sheets for use in accordance with aspects ofthe present invention will be described below. Unless otherwisespecified, the percentages of the components are on a mass basis.

C: 0.08% or more and less than 0.20%

A C content of 0.08% or more is required to sufficiently promote theformation of Si oxides containing Fe and/or Mn by lowering oxygenpotential through decarbonization. A C content of less than 0.20%results in good workability. Thus, the C content is 0.08% or more andless than 0.20%. The C content is preferably 0.08% or more, morepreferably 0.10% or more.

Si: 0.1% to 3.0%

Si is the most important element to improve the mechanicalcharacteristics of steel sheets. The Si content should be 0.1% or more.However, a Si content of more than 3.0% results in insufficientsuppression of the formation of SiO₂-based oxides at the interfacebetween ferrite and oxides, making it difficult to have the oxidecontent required to suppress the formation of uncoated areas. Thus, theSi content ranges from 0.1% to 3.0%.

Mn: 0.5% to 3.0%

Mn is a solid-solution strengthening element and is effective inreinforcing steel sheets. The Mn content should be 0.5% or more. A Mncontent of more than 3.0% results in poor weldability and adhesion tothe coating and makes it difficult to maintain the strength ductilitybalance. Thus, the Mn content ranges from 0.5% to 3.0%.

P: 0.001% to 0.10%

P retards the precipitation and phase transformation of cementite. The Pcontent is 0.001% or more. However, a P content of more than 0.10%results in poor weldability and adhesion to the coating. Furthermore,this retards alloying, increases the alloying temperature, and decreasesductility. Thus, the P content ranges from 0.001% to 0.10%.

Al: 0.01% to 3.00%

Al and Si are complementary to each other. Al is an inevitablecontaminant in the steel production process. The lowest Al content is0.01%. An Al content of more than 3.00% makes it difficult to suppressthe formation of external oxides and results in poor adhesiveness of thecoated layer. Thus, the Al content ranges from 0.01% to 3.00%.

S: 0.200% or less

S is an element that is inevitably introduced in the steel productionprocess. However, a high S content results in poor weldability. Thus,the S content is 0.200% or less.

In some embodiments of the present invention, in addition to thesecomponents, Mo and/or Cr may be further contained.

Mo: 0.01% to 1.00%

Mo is an element that controls the strength ductility balance. The Mocontent may be 0.01% or more. Mo is effective in promoting internaloxidation of Si and Al and suppressing surface enrichment of Si and Al.However, a Mo content of more than 1.00% may result in increased costs.Thus, when Mo is contained, the Mo content preferably ranges from 0.01%to 1.00%.

Cr: 0.01% to 1.00%

Cr is an element that controls the strength ductility balance. The Crcontent may be 0.01% or more. Like Mo, Cr is effective in promotinginternal oxidation of Si and Al and suppressing surface enrichment of Siand Al. However, a Cr concentration of more than 1.00% results insurface enrichment of Cr and poor adhesion to the coating andweldability. Thus, when Cr is contained, the Cr content preferablyranges from 0.01% to 1.00%.

In some embodiments of the present invention, in addition to thesecomponents, the following elements may be contained in a manner thatdepends on the desired characteristics.

Nb: 0.005% to 0.20%

Nb is an element that controls the strength ductility balance. The Nbcontent may be 0.005% or more. However, a Nb content of more than 0.20%may result in increased costs. Thus, when Nb is contained, the Nbcontent preferably ranges from 0.005% to 0.20%.

Ti: 0.005% to 0.20%

Ti is an element that controls the strength ductility balance. The Ticontent may be 0.005% or more. However, a Ti content of more than 0.20%may result in poor adhesion to the coating. Thus, when Ti is contained,the Ti content preferably ranges from 0.005% to 0.20%.

Cu: 0.01% to 0.50%

Cu is an element that promotes the formation of a retained austenitephase. The Cu content may be 0.01% or more. However, a Cu content ofmore than 0.5% may result in increased costs. Thus, when Cu iscontained, the Cu content preferably ranges from 0.01% to 0.50%.

Ni: 0.01% to 1.00%

Ni is an element that promotes the formation of a retained austenitephase. The Ni content may be 0.01% or more. However, a Ni content ofmore than 1.00% may result in increased costs. Thus, when Ni iscontained, the Ni content preferably ranges from 0.01% to 1.00%.

B: 0.0005% to 0.010%

B is an element that promotes the formation of a retained austenitephase. The B content may be 0.0005% or more. However, a B content ofmore than 0.010% may result in poor adhesion to the coating. Thus, whenB is contained, the B content preferably ranges from 0.0005% to 0.010%.

The remainder is Fe and incidental impurities.

Internal oxides and a decarburized layer, which are important in aspectsof the present invention, present on the ferrite side from the interfacebetween ferrite and a galvanized layer will be described below.

In accordance with aspects of the present invention, there are aninternal oxidation layer having a thickness of 5 μm or less and adecarburized layer having a thickness of 20 μm or less on the ferriteside from the interface between ferrite and a galvanized layer, and 50%or more by area of the internal oxidation layer is composed of a Sioxide containing Fe and/or Mn.

Before hot-dip galvanizing treatment, a cold-rolled steel sheet isheated in a direct heating type furnace and then in a reducingatmosphere. In the direct heating type furnace, the surface of the steelsheet is heated with a direct heating burner. When the steel sheetcontains sufficient carbon, decarbonization as represented by thefollowing formula (1) occurs inside the steel sheet simultaneously withoxidation of the surface of the steel sheet due to heating with thedirect heating burner. This decarbonization lowers oxygen potential inthe steel sheet.[Formula 1]C+½O₂→CO  (1)

A decrease in oxygen potential promotes an equilibrium reaction by whicha Si oxide containing Fe and/or Mn is formed from SiO₂ at the interfacebetween external oxides and ferrite (see, for example, the followingformula (2)).[Formula 2]Fe_(2X)Mn_(2-2X)O₃+SiO₂⇄Fe_(2X)Mn_(2-2X)SiO₄+0.5O₂↓  (2)

In accordance with aspects of the present invention, low oxygenpotential due to decarbonization promotes the reaction represented bythe formula (2) and efficiently promotes the formation of Si oxidescontaining Fe and/or Mn. Ion diffusion in Si oxides containing Fe and/orMn is much faster than in SiO₂, and even short-time heating, such asheating with a direct heating burner, can achieve the oxide contentrequired to suppress the formation of uncoated areas. SiO₂ restricts iondiffusion and makes it difficult to form external oxides required tosuppress the formation of uncoated areas by short-time heating, such asheating with a direct heating burner. In accordance with aspects of thepresent invention, decarbonization promotes a reaction from SiO₂ to Sioxides containing Fe and/or Mn at the interface between ferrite andexternal oxides and thereby forms external oxides required to suppressthe formation of uncoated areas.

Internal oxides formed by oxidation are exposed on a portion of thesurface of annealed steel sheets not covered with a reduced Fe layer.SiO₂ on the surface acts as a starting point of an uncoated area thatrepels molten zinc and significantly affects the appearance of thegalvanized surface. In accordance with aspects of the present invention,a reaction from SiO₂ to Si oxides containing Fe and/or Mn is promoted asthe oxygen potential decreases. Because Si oxides containing Fe and/orMn have higher wettability to molten zinc than SiO₂, Si oxidescontaining Fe and/or Mn even exposed on the surface rarely act asstarting points of uncoated areas. This therefore suppresses theformation of uncoated areas.

An excessive amount of internal oxides in steel sheets reduces theadhesiveness of a Zn coating. This is because a difference between thecoefficient of thermal expansion of internal oxides and the coefficientof thermal expansion of ferrite causes a void at the interface betweenthe internal oxides and ferrite in the production process, and the voidacts as a starting point for the propagation of cracks. Si oxidescontaining Fe and/or Mn have a coefficient of thermal expansion closerto that of Fe than SiO₂ and therefore rarely cause a void between theinternal oxides and ferrite. Thus, Si oxides containing Fe and/or Mnimprove the adhesiveness of a Zn coating as compared with SiO₂.

The internal oxidation layer has a thickness of 5 μm or less on theferrite side from the interface between ferrite and a galvanized layer.A thickness of more than 5 μm may result in internal defects calledblack stains in cross-sectional observation. Furthermore, an excessiveamount of internal oxidation decreases the adhesiveness of a Zn coating.The decarburized layer has a thickness of 20 μm or less on the ferriteside from the interface between the ferrite and the galvanized layer. Athickness of more than 20 μm makes it difficult to form a retainedaustenite phase and reduces the advantage of the addition of Si inmechanical characteristics.

50% or more by area of the internal oxidation layer is composed of a Sioxide containing Fe and/or Mn. An area percentage of less than 50% isinsufficient to improve the coating appearance and adhesion by theformation of Si oxides containing Fe and/or Mn. FIG. 1 is an exemplaryconcentration profile of carbon and oxygen in ferrite in a hot-dipgalvanized steel sheet according to aspects of the present invention.

Si oxides containing Fe and/or Mn can be identified by the compositionanalysis of Si, Mn, and Fe in the oxides by EDX of a cross-sectionalmicrostructure observed by SEM. EPMA element mapping or TEM electrondiffraction images may also be used for the identification. The areapercentage in accordance with aspects of the present invention refers tothe percentage of Si oxides containing Fe and/or Mn in the entireinternal oxidation layer. The area percentage is determined by measuringthe concentrations of Si, Mn, and Fe in internal oxides contained inferrite by EPMA element mapping of a cross-sectional microstructure. Anoxide having a Si content of 95% or more is considered to be SiO₂, andan oxide having a Si content of less than 95% is considered to be a Sioxide containing Fe and/or Mn. The area percentages of the internaloxidation layer, decarburized layer, and Si oxides containing Fe and/orMn in accordance with aspects of the present invention can be controlledby adjusting the annealing conditions, the C content of steel, and theSi content of steel.

A method for producing a hot-dip galvanized steel sheet according toaspects of the present invention will be described below.

Steel having the composition described above is hot-rolled and thencold-rolled to form a steel sheet. The steel sheet is then subjected toannealing and hot-dip galvanizing treatment in continuous hot-dipgalvanizing equipment including a direct heating type furnace equippedwith a direct heating burner. If necessary, the hot-dip galvanizingtreatment may be followed by alloying treatment.

Hot Rolling

The hot rolling may be performed under typical conditions.

Pickling

The hot rolling is preferably followed by pickling treatment. Mill scaleformed on the surface is removed in a pickling process before coldrolling. The pickling conditions are not particularly limited.

Cold Rolling

The cold rolling is preferably performed at a rolling reduction in therange of 30% to 90%. A rolling reduction of less than 30% often resultsin poor mechanical characteristics due to slow recrystallization. On theother hand, a rolling reduction of more than 90% results in not onlyincreased rolling costs but also poor coating characteristics due toincreased surface enrichment in an annealing operation.

The annealing conditions for the formation of internal oxides and adecarburized layer, which are important in accordance with aspects ofthe present invention, will be described below. The annealing is aheat-treatment process including heating in a direct heating typefurnace and then heating in a reducing atmosphere.

Heating a steel sheet to a final surface temperature in the range of600° C. to 800° C. in an atmosphere containing a combustible gas and acombustion-supporting gas, the concentration of CO and hydrocarbon gasin the combustible gas being 60% or less by volume, the concentration ofO₂ in the combustion-supporting gas ranging from 20% to 50% by volume

In accordance with aspects of the present invention, a steel sheet isheated in a direct heating type furnace after the cold rolling. Morespecifically, the surface of the steel sheet is heated with a directheating burner in a direct heating type furnace. The steel sheet isheated to a final surface temperature in the range of 600° C. to 800° C.final surface temperature of 600° C. or less results in a deficiency ofoxides required to suppress the formation of uncoated areas. On theother hand, a final surface temperature of 800° C. or more results in anexcessive amount of oxides and causes defects called indentation flawson the surface. Thus, the final surface temperature ranges from 600° C.to 800° C.

Heating in a direct heating type furnace is performed in an atmospherehaving low carbon and oxygen potential. More specifically, the steelsheet is heated by burning a combustible gas and a combustion-supportinggas with a direct heating burner in a direct heating type furnace, thecombustible gas containing CO, a hydrocarbon gas, and a remainder, theCO and hydrocarbon gas constituting 60% or less by volume in total, theremainder being H₂, N₂, and incidental impurities, thecombustion-supporting gas containing O₂ and a remainder, the O₂constituting 20% to 50% by volume, the remainder being N₂ and incidentalimpurities. Under conditions outside the conditions described above,decarbonization cannot sufficiently lower the oxygen potential at theinterface between ferrite and oxides.

Heating a steel sheet at a soaking temperature in the range of 630° C.to 850° C. in an atmosphere having a hydrogen concentration in the rangeof 3% to 25% by volume and a water vapor concentration of 0.070% or lessby volume and containing N₂ and incidental impurities as a remainder

After heating with the direct heating burner, the steel sheet is heated(annealed) at a soaking temperature in the range of 630° C. to 850° C.in an atmosphere having a hydrogen concentration in the range of 3% to25% by volume and a water vapor concentration of 0.070% or less byvolume and containing N₂ and incidental impurities as a remainder. Thisis performed in order to reduce the surface of the steel sheet. Forsufficient reducing ability, the hydrogen concentration should be 3% byvolume or more. However, a hydrogen concentration of 25% by volume ormore results in high operating costs. A water vapor concentration of0.070% by volume or more results in promoted decarbonization by H₂O inan annealing operation, and the decarburized layer has a thickness ofmore than 20 μm. An excessively thick decarburized layer makes itdifficult to form a retained austenite phase and reduces the advantageof the addition of Si. Thus, the annealing atmosphere has a hydrogenconcentration in the range of 3% to 25% by volume and a water vaporconcentration of 0.070% or less by volume.

In the atmosphere described above, the steel sheet is heated at asoaking temperature in the range of 630° C. to 850° C. for reductionannealing. A final steel sheet temperature of 630° C. or less results inpoor mechanical characteristics due to slow recrystallization. A finalsteel sheet temperature of more than 850° C. results in the formation ofuncoated areas due to surface enrichment of Si and the like.

The annealing is followed by hot-dip galvanizing treatment. The hot-dipgalvanizing treatment may be followed by alloying treatment, ifnecessary.

The temperatures of a Zn bath in the hot-dip galvanizing treatment andalloying treatment preferably range from 440° C. to 550° C. A bathtemperature of less than 440° C. may disadvantageously result insolidification of Zn due to large variations in the bath temperature. Onthe other hand, a bath temperature of more than 550° C. results in rapidevaporation of components in the Zn bath and increased operating costsor pollution of the operating environment due to evaporation from the Znbath. This also tends to result in over-alloying because alloyingproceeds while the steel sheet is immersed in the Zn bath.

Without alloying treatment, it is desirable that the concentration of Alin the bath range from 0.14% to 0.24% by mass. An Al concentration ofless than 0.14% by mass results in an uneven appearance due to an Fe—Znalloying reaction in a coating operation. An Al concentration of morethan 0.24% by mass results in poor weldability because a thick Fe—Alalloy layer is formed at the interface between the galvanized layer andferrite during the coating treatment. This high concentration of Al inthe bath also results in deposition of a large amount of Al oxide filmon the surface of the steel sheet and a very poor surface appearance.

When the hot-dip galvanizing treatment is followed by alloyingtreatment, it is desirable that the concentration of Al in the bathrange from 0.10% to 0.20% by mass. An Al concentration of less than0.10% by mass results in poor adhesion to the coating because a hard andbrittle Fe—Zn alloy layer is formed at the interface between thegalvanized layer and ferrite in a coating operation. An Al concentrationof more than 0.20% by mass results in poor weldability because a thickFe—Al alloy layer is formed at the interface between the galvanizedlayer and ferrite immediately after immersion in the bath.

Mg may be added to the Zn bath in order to improve corrosion resistance.

When the hot-dip galvanizing treatment is followed by alloying treatmentas required, the alloying temperature is preferably 460° C. or more andless than 570° C. An alloying temperature of 460° C. or less results ina slow alloying reaction. An alloying temperature of 570° C. or moreresults in poor coating characteristics because a thick, hard andbrittle Fe—Zn alloy layer is formed at the coated layer/ferriteinterface. The amount of coating is not particularly limited. The amountof coating is preferably 10 g/m² or more in terms of corrosionresistance and the control of the amount of coating and is preferably120 g/m² or less in terms of workability and from an economic point ofview.

Example 1

Aspects of the present invention will be more specifically described inthe following example.

A slab having a steel composition listed in Table 1 was heated in afurnace at 1260° C. for 60 minutes, was hot-rolled to 2.8 mm, and wascoiled at 540° C. The steel sheet was then pickled to remove mill scaleand was cold-rolled to 1.4 mm at a rolling reduction of 50%. The steelsheet was then subjected to heat treatment (annealing) under theconditions listed in Table 2 in a CGL having a direct heating (DFF) typeheating zone. Subsequently, the steel sheet was immersed in a Zn bathcontaining Al at 460° C. for hot-dip galvanizing treatment to produce ahot-dip galvanized steel sheet (coating type: GI). Some of the steelsheets were subjected to alloying treatment after the hot-dipgalvanizing treatment to produce galvannealed steel sheets (coatingtype: GA). The concentration of Al in the bath ranged from 0.10% to0.20% by mass, and the amount of coating was adjusted to be 45 g/m² bygas wiping. The alloying treatment was performed at a temperature in therange of 550° C. to 560° C.

TABLE 1 Components/mass % Steel type C Si Mn P Al S Mo Cr Nb Ti Cu Ni BRemarks A 0.10 1.2 0.7 0.05 0.60 0.003 — — — — — — — Within scope ofinvention B 0.08 0.2 0.4 0.01 1.00 0.006 — — — — — — — Within scope ofinvention C 0.15 2.0 1.0 0.08 0.03 0.001 0.04 — — — — — Within scope ofinvention D 0.19 0.8 1.5 0.02 0.07 0.020 0.01 — — — — — — Within scopeof invention E 0.12 0.5 2.2 0.04 0.80 0.050 0.02 0.08 — — — — — Withinscope of invention F 0.09 0.9 3.0 0.06 0.05 0.041 — 0.06 0.04 — — — —Within scope of invention G 0.18 2.4 0.6 0.04 0.20 0.003 0.15 — 0.080.02 — — — Within scope of invention H 0.13 0.3 1.5 0.08 0.08 0.007 0.050.04 — — 0.20 — — Within scope of invention I 0.14 1.8 0.8 0.03 0.600.008 0.07 0.06 0.10 0.03 0.01 0.60 — Within scope of invention J 0.110.6 2.2 0.02 1.20 0.023 0.50 0.20 — 0.10 — — 0.002 Within scope ofinvention K 0.25 1.1 0.8 0.06 0.30 0.026 0.50 0.02 — — — — — Outsidescope of invention L 0.04 2.3 1.7 0.03 0.10 0.001 0.02 — — 0.05 0.25 — —Outside scope of invention M 0.15 3.5 3.1 0.08 1.50 0.300 — 0.08 0.07 —0.02 — — Outside scope of invention N 0.35 2.0 1.0 0.02 3.20 0.001 1.200.45 — 0.12 — — 0.001 Outside scope of invention O 0.05 0.8 1.6 0.040.40 0.015 1.35 2.50 0.04 0.07 0.05 0.02 0.001 Outside scope ofinvention P 0.52 0.7 1.8 0.04 0.50 0.015 0.35 1.50 0.04 — 0.03 0.050.001 Outside scope of invention

The surface appearance and adhesiveness of the coating of the hot-dipgalvanized steel sheets thus produced were evaluated by the followingmethod. After the galvanized layer was removed, the composition in thedepth direction was analyzed with a glow discharge spectrometer (GDS),and the thicknesses of the internal oxidation layer and decarburizedlayer were determined. More specifically, a region in which at leasttwice the average of oxygen peaks observed at a depth in the range of 50to 60 μm was detected was considered to be an internal oxidation layer.A region in which less than half the average of carbon peaks observed ata depth in the range of 50 to 60 μm was detected was considered to be adecarburized layer. The area percentage of Si oxides containing Feand/or Mn was determined by cross-sectional EPMA element mapping. Morespecifically, the concentrations of Si, Mn, and Fe in internal oxidescontained in the ferrite were determined. An oxide having a Si contentof 95% or more was considered to be SiO₂, and an oxide having a Sicontent of less than 95% was considered to be a Si oxide containing Feand/or Mn.

The following are evaluation criteria for the surface appearance andadhesiveness of the coating.

(1) Surface Appearance

A 300 mm×300 mm area was visually inspected. The surface appearance wasrated according to the following criteria:

◯ (Circle): No uncoated area or indentation flaws

Δ (Triangle): Generally good except for a few uncoated areas orindentation flaws

X (Cross): Poor appearance with uncoated areas or indentation flaws

(2) Adhesiveness of Coating

A cellophane adhesive tape was applied to a coated surface. The tapesurface was bent 90° C. and bent back. A cellophane adhesive tape havinga width of 24 mm was applied to the inside of the processed portion(compressed side) parallel to the bent portion and was removed. Theamount of zinc deposited on a portion of the cellophane adhesive tapehaving a length of 40 mm was measured as a Zn count by a fluorescentX-ray method and was converted into the amount of peeled zinc per unitlength (1 m), which was evaluated according to the following criteria.The mask diameter was 30 mm, the accelerating voltage and acceleratingcurrent of fluorescent X-rays were 50 kV and 50 mA, and the measurementtime was 20 seconds.

◯ (Circle): The Zn count was 0 or more and less than 5000.

Δ (Triangle): The Zn count was 5000 or more and less than 10000.

X (Cross): The Zn count was 10000 or more.

Table 2 shows the results.

TABLE 2 Heat treatment (annealing) conditions Final steel CO and O₂ inSteel sheet surface hydrocarbon in combustion- Soaking Inside of ferritesheet Steel temperature/ combustible supporting temperature/Decarburized No. type ° C. gas/vol % gas/vol % H₂/vol % H₂O/vol % ° C.layer/μm 1 A 650 50 20 5 0.065 820 12 2 A 740 56 25 20 0.050 810 15 3 A820 50 25 15 0.040 810 30 4 A 550 45 21 8 0.020 815 10 5 B 780 40 21 50.010 850 16 6 B 690 35 40 5 0.045 800 8 7 B 790 55 30 1 0.030 820 26 8B 740 80 35 10 0.050 800 11 9 C 705 58 40 6 0.050 800 10 10 C 580 60 105 0.020 805 30 11 D 650 50 45 10 0.010 820 14 12 D 740 55 21 12 0.150820 40 13 E 700 40 21 9 0.020 790 5 14 E 850 40 40 16 0.050 810 32 15 F620 30 25 8 0.060 820 19 16 F 500 55 25 8 0.040 820 12 17 G 650 52 35 200.010 820 5 18 H 710 54 21 12 0.031 820 13 19 I 700 40 40 13 0.026 820 720 J 685 48 30 16 0.040 820 18 21 K 695 52 30 14 0.060 840 35 22 L 72045 22 16 0.015 820 5 23 M 770 58 36 8 0.060 820 8 24 N 735 60 25 150.070 820 25 25 O 680 45 21 12 0.030 820 15 26 P 740 56 30 20 0.055 82030 Inside of ferrite Area percentage Steel Internal of Si oxides sheetoxidation containing Fe Coating Surface No. layer/μm and/or Mn/% typeappearance Adhesion Remarks 1 0.5 56.0 GI ∘ ∘ Example 2 1.2 74.0 GA ∘ ∘Example 3 5.6 55.0 GI Δ x Comparative example 4 0.5 30.0 GI x ΔComparative example 5 3.1 64.0 GA ∘ ∘ Example 6 2.3 60.0 GI ∘ ∘ Example7 4.6 36.0 GA x Δ Comparative example 8 0.3 20.0 GA Δ x Comparativeexample 9 1.9 80.0 GI ∘ ∘ Example 10 0.5 18.0 GA x x Comparative example11 2.8 82.0 GA ∘ ∘ Example 12 5.2 62.0 GI Δ x Comparative example 13 3.471.0 GA ∘ ∘ Example 14 3.5 45.0 GA Δ x Comparative example 15 1.5 59.0GI ∘ ∘ Example 16 0.8 25.0 GA x Δ Comparative example 17 1.2 61.0 GA ∘ ∘Example 18 2.4 72.0 GA ∘ ∘ Example 19 4.2 85.0 GI ∘ ∘ Example 20 4.682.0 GA ∘ ∘ Example 21 7.0 60.0 GA Δ x Comparative example 22 2.6 34.0GI x Δ Comparative example 23 2.3 28.0 GA x x Comparative example 24 4.065.0 GA Δ x Comparative example 25 1.7 30.0 GA x x Comparative example26 5.8 75.0 GA Δ x Comparative example

The results in Table 2 were summarized in FIG. 2. FIG. 2(a) illustratesthe results with respect to the surface appearance, and FIG. 2(b)illustrates the results with respect to the adhesiveness of the coating.The surface of hot-dip galvanized steel sheets (examples) having a 5 μmor less internal oxidation layer and a 20 μm or less decarburized layer,50% or more by area of the internal oxidation layer being composed of aSi oxide containing Fe and/or Mn, had a good appearance and goodadhesion to the coating.

INDUSTRIAL APPLICABILITY

Steel sheets according to aspects of the present invention have goodmechanical characteristics, a good appearance of coating, and goodadhesion and are therefore expected to be used in a wide range ofapplications, such as automobiles, household electrical appliances, andconstruction materials.

The invention claimed is:
 1. A hot-dip galvanized steel sheet having abase steel sheet and a galvanized layer, the base steel sheet having acomposition comprising, on a mass basis: C: 0.08% or more and less than0.20%, Si: 0.1% to 3.0%, Mn: 0.5% to 3.0%, P: 0.001% to 0.10%, Al: 0.01%to 3.00%, and S: 0.200% or less, a remainder being Fe and incidentalimpurities, wherein the base steel sheet includes an internal oxidationlayer and a decarburized layer, the internal oxidation layer having athickness of 0.5 μm or more and 5 μm or less on a ferrite side from aninterface between ferrite and a galvanized layer, the decarburized layerhaving a thickness of 5 μm or more and 20 μm or less on the ferrite sidefrom the interface between the ferrite and the galvanized layer, and 50%or more by area of the internal oxidation layer is composed of a Sioxide containing Fe and/or Mn represented by Fe_(2X)Mn_(2-2X)SiO_(Y),wherein X ranges from 0 to 1, and Y is 3 or
 4. 2. The hot-dip galvanizedsteel sheet according to claim 1, the base steel sheet furthercomprising Mo: 0.01% to 1.00% and/or Cr: 0.01% to 1.00% on a mass basis.3. The hot-dip galvanized steel sheet according to claim 2, the basesteel sheet further comprising at least one of Nb: 0.005% to 0.20%, Ti:0.005% to 0.20%, Cu: 0.01% to 0.50%, Ni: 0.01% to 1.00%, and B: 0.0005%to 0.010% on a mass basis.
 4. The hot-dip galvanized steel sheetaccording to claim 2, wherein the galvanized layer is a galvannealedlayer.
 5. The hot-dip galvanized steel sheet according to claim 1, thebase steel sheet further comprising at least one of Nb: 0.005% to 0.20%,Ti: 0.005% to 0.20%, Cu: 0.01% to 0.50%, Ni: 0.01% to 1.00%, and B:0.0005% to 0.010% on a mass basis.
 6. The hot-dip galvanized steel sheetaccording to claim 5, wherein the galvanized layer is a galvannealedlayer.
 7. The hot-dip galvanized steel sheet according to claim 1,wherein the galvanized layer is a galvannealed layer.
 8. A method forproducing a hot-dip galvanized steel sheet, comprising in sequence thesteps of: hot-rolling steel having the composition according to claim 1,cold-rolling the resulting hot-rolled steel sheet; heating the steelsheet to a final surface temperature in the range of 600° C. to 800° C.by burning a combustible gas and a combustion-supporting gas with adirect heating burner in a direct heating type furnace, the combustiblegas containing CO, a hydrocarbon gas, and a remainder, the CO andhydrocarbon gas constituting 60% or less by volume in total, theremainder being H₂, N₂, and incidental impurities, thecombustion-supporting gas containing O₂ and a remainder, the O₂constituting 20% to 50% by volume, the remainder being N₂ and incidentalimpurities; heating the steel sheet at a soaking temperature in therange of 630° C. to 850° C. in an atmosphere having a hydrogenconcentration in the range of 3% to 25% by volume and a water vaporconcentration of 0.070% or less by volume and containing N₂ andincidental impurities as a remainder; and hot-dip galvanizing the steelsheet.
 9. The method for producing a hot-dip galvanized steel sheetaccording to claim 8, further comprising alloying the zinc coating afterthe hot-dip galvanizing.
 10. A method for producing a hot-dip galvanizedsteel sheet, comprising in sequence the steps of: hot-rolling steelhaving the composition according to claim 2, cold-rolling the resultinghot-rolled steel sheet; heating the steel sheet to a final surfacetemperature in the range of 600° C. to 800° C. by burning a combustiblegas and a combustion-supporting gas with a direct heating burner in adirect heating type furnace, the combustible gas containing CO, ahydrocarbon gas, and a remainder, the CO and hydrocarbon gasconstituting 60% or less by volume in total, the remainder being H₂, N₂,and incidental impurities, the combustion-supporting gas containing O₂and a remainder, the O₂ constituting 20% to 50% by volume, the remainderbeing N₂ and incidental impurities; heating the steel sheet at a soakingtemperature in the range of 630° C. to 850° C. in an atmosphere having ahydrogen concentration in the range of 3% to 25% by volume and a watervapor concentration of 0.070% or less by volume and containing N₂ andincidental impurities as a remainder; and hot-dip galvanizing the steelsheet.
 11. A method for producing a hot-dip galvanized steel sheet,comprising in sequence the steps of: hot-rolling steel having thecomposition according to claim 5, cold-rolling the resulting hot-rolledsteel sheet; heating the steel sheet to a final surface temperature inthe range of 600° C. to 800° C. by burning a combustible gas and acombustion-supporting gas with a direct heating burner in a directheating type furnace, the combustible gas containing CO, a hydrocarbongas, and a remainder, the CO and hydrocarbon gas constituting 60% orless by volume in total, the remainder being H₂, N₂, and incidentalimpurities, the combustion-supporting gas containing O₂ and a remainder,the O₂ constituting 20% to 50% by volume, the remainder being N₂ andincidental impurities; heating the steel sheet at a soaking temperaturein the range of 630° C. to 850° C. in an atmosphere having a hydrogenconcentration in the range of 3% to 25% by volume and a water vaporconcentration of 0.070% or less by volume and containing N₂ andincidental impurities as a remainder; and hot-dip galvanizing the steelsheet.